Titanium dioxide has a wide variety of uses, e.g., as a pigment, a catalyst or a photoconductor, as well as other uses. Processes for the preparation of titanium dioxide are known in the art. In one such process, the ore ilmenite, containing titanium and iron, is treated with sulfuric acid, and the resulting solution is thermally hydrolyzed and then calcined in the presence of salts and/or orienting nuclei. Another such process is based on the chlorination of mineral rutile and/or enriched titanium-containing ores, to form titanium tetrachloride, followed by the purification of the titanium tetrachloride and its oxidation in the presence of other chlorides, especially AlCl.sub.3.
The titanium dioxide provided by such processes typically is in the form of irregularly shaped prismatic or spheroidal particles, also having, in general, a broad size distribution. As is known, a broad granulometric distribution of titanium dioxide particles detracts from the usefulness of this material in many commercial applications. For instance, the color purity and optical performance of titanium dioxide pigments, either alone or in admixture with other pigments, may be adversely affected. Moreover, the titanium dioxide particles from these processes are often aggregated, and as a rule long and expensive procedures for breaking down the aggregates into smaller, individual particles are required.
On the other hand, it is known that the hydrolysis of an aqueous solution of titanium tetrachloride, leads to the formation of titanium dioxide as needle-shaped particles having a predominantly rutile crystal structure.
E. Matijevic et al, in the Journal of Colloid and Interface Science, Vol. 61, page 302 (1977), describe the preparation of non-aggregated spherical particles of titanium dioxide, in a narrow size distribution, by the hydrolysis of a solution of titanium tetrachloride at elevated temperatures, in the presence of sulfuric acid (sulfate ions). The method is not entirely satisfactory, however, because very long processing times are required, yields are low and very small particles, e.g., about 0.2 .mu.m or less, which are useful in pigment manufacture, are not obtained. Moreover, not all of the titanium dioxide particles appear as regular spheres.
It is desirable that titanium dioxide particles have the following characteristics:
(i) a narrow size distribution; PA0 (ii) the substantial absence of aggregation; and PA0 (iii) a substantially uniform spherical shape. PA0 (a) nucleation, PA0 (b) evaporation, PA0 (c) condensation, PA0 (d) reheating, and PA0 (e) recondensation.
These characteristics permit the application of rigorous light scattering principles to the optical behaviour of the particles, and, thus, to determine in advance the optimum diameter of the titanium dioxide for its various uses.